My blender game is not rendering properly, some faces not showing and materias don't show up correctly
So, you have a model in your game. Some of it's faces seem to have disappeared:


This is probably due to the normals of the model having been scrambled somehow. To check if this is indeed the case, you need to enable the normals display of your faces in edit-mode. To do this, hit N on your keyboard to open up the properties tab on the right of the viewport. Scroll down to 'Mesh Display' and under 'Normals', tick the symbol that displays a face on it:


If you now look at your model, you should see little blue lines stick out of its faces. All lines that are on the outside of the model are attached to faces that have its normals on the outside of the model. This is good. All lines that are on the inside of the model are of course wrong normals. These will display as basically nothing in your game because you are looking at the back of the face.

To fix this, make sure to be in edit-mode and select everything by pressing A until the whole mesh is selected. Then, go to the bottom of your viewport, hit 'Mesh', then 'Normals' and choose 'Recalculate outside'. You can also do this by hitting Ctrl+N:
 
This handy tool will recalculate your mesh to make every selected face face outwards. You should see all the lines stick outwards now:


Now, of course your last step is to get out of edit-mode and run your game: